Tuesday, March 23, 2010

COLOUR ABERRATION IN TIGER: ITS BIOLOGICAL AND CONSERVATION IMPLICATIONS

Based on:SINGH, L. A. K. (1999): Born Black: The Melanistic Tiger in India. WWF-India, New Delhi, 1999.66pp.While talking about the body colour of Tiger, we normally mean the yellow or tawny body with black stripes. Variations within normal colouration are attributed to geographical regions, the forest-habitat, and perhaps the season, as well. In this context, the white Tigers are also discussed and admired as a variation. Tigers of other colouration are extremely rare and have not been discussed in as much detail as the white tigers have been. However, there are at least fourteen known types of body colouration in Tiger, and there could be a larger intermediary range within these (Fig.1).

In the recent memory from Similipal Tiger Reserve, Orissa, aberrantly coloured tigers were first seen in early 1970s. However, most reports since then have been regarded as freaks or as the observer's imagination. Thus they remained ignored or undocumented. An incident of July 1993 in the valley of River Bhandan led to a detailed investigation about the occurrence of the phenomenon of aberrant colouration in the population gene pool of Panthera tigris as a species.

The incidentOn the 21st July 1993 around 10 AM a young melanistic tigress was killed by a boy in “self-defense” with arrows. It occurred in the village Podagad in the western periphery of the Similipal Tiger Reserve. The major peculiarity in the body colou­ration was that on its back the black colour was preponderant.

The young tigress had injured "4-5 goats" during the previ­ous one week. Every time it injured a goat the victim was taken away by the villagers. On dt. 20.7.93 night the tigress entered into the cow shed of Sri Surai Besra, 67 years. On hearing some sound Surai went towards the cow shed. The tigress charged at and injured Surai on his face. In the mean time, other members of the family woke up and on shouting the tigress retreated away. The next morning at about 10 am Salku, Surai's son sighted the tigress in the adjoin­ing maize field. The tigress charged towards Salku who ran into his house and from there he aimed at and killed the tigress with three arrows. Later, there have been many reports of melanistic sightings in Similipal. In March 1997, a melanistic tiger was sighted in Satkoshia Gorge Sanctuary in the (former) Dhenkanal district in Orissa.

Earlier, the skin of a melanistic tiger was recovered from smugglers in October 1992 in south Delhi. The skin measured eight and a half foot (259cm) and was displayed at the National Museum of Natural History, New Delhi in February 1993. The source of the skin is not known.

Following the incident of July 1993, information on tigers having aberrant colouration were collected from Similipal and elsewhere. These were supported with data from tiger-rearing facilities in Florida, USA and additional information from published literature.

The colour of tiger

Normally, the tiger's coat displays a combination of three colours- white, yellow and black. The background colour of the body is controlled by a set of 'agouti' genes and their alleles. 'Tabby genes' and their alleles control stripes. Built within the two series (background and stripe) some genes determine the location-to-location and quantum of expression of three main skin colours white, yellow and black. The absence of any of these colours or genetic suppression (epistasis) of the effects of genes responsible for their expression lead to colour variation in tiger.

Any species of patterned cat is more likely to produce colour aberration if the local population is 'evolutionary old' or its population has reduced in number to such an extent that it leads to inbreeding and encourages 'inbreeding depression.' The reports of aberrants are not many because of various reasons. While the dismissals of observations have discouraged fresh reporting, sighting of an aberrant Tiger is an extremely rare event, and there is early elimination of aberrants from the population of normal individuals.

The appearance of Tigers with aberrant colouration can be expected as a regular but extremely rare natural phenomenon. Only in populations where inbreeding has a longer and stronger influence, the appearance of aberrants would be more frequent.

Biological ImplicationsAll colours other than the “normal” are considered to have inappropriate adaptive value in the wild state. Besides, colour appears related to the body-size. While the normal colour of a Tiger and its size are the best compromise for Panthera tigris in the wild and are evolution-tested through Natural Selection, the White Tigers have a large body while the Black Tigers are diminutive.

Following thousands of years of evolution, struggle for existence and natural selection, a species is not meant to be an aberrant. It is against the natural order. Therefore, natural elimination of aberrants from the population is effected through (a) unsuitable structural or physiological organisation, and/or (b) early separation from the mother. This phenomenon may not be clear in captivity where rearing conditions provide environment-enrichment and the living style is without competition or struggle. Adaptive values of soft features, like body colour, have little significance in captivity.

Conservation ImplicationsThe normal colour of a Tiger and its size are not only best suited to the species in the wild but this combination has also the public appeal that has been so essential for Tiger Conservation. Tiger shall lose public appeal if 'inbreeding depression' and evolutionary processes lead the species at any time towards increased melanism or aberrant colouration. The discovery of polymorphism and increased possibility of 'inbreeding depression' calls for greater attention to tiger-conservation because evolution may be proceeding towards enhanced melanism in tiger, as it is seen for the black leopards. Such shift is likely to occur first with marginal populations which carry a lighter genetic load with a small number of lethals and heterozygosity.

Further, the threats to Tiger continues because of growing human population, and it is not known in what exact direction the evolution of Tiger is proceeding now. The possibilities of appearance of more numbers of aberrants cannot be ruled out if populations become small, fragmented and isolated. Therefore, large and contiguous patches of forest, if necessary with corridors, may improve 'genetic exchange' and reduce genetic erosion. Conservation of Tiger requires to be aimed at reducing the possibility of genetic erosion in the wild through habitat improve­ment.